It is desirable in optical wavelength-division-multiplexing networks to have inexpensive light modulators that have high contrast and wide optical bandwidths. In certain cases, such as audio and video transmission, these modulators need only operate at frequencies up to several megahertz.
A modulation device particularly well suited for the above application is a surface normal micromechanical modulator. This device may be described as having a variable air gap defined by two layers of material. Typically, surface normal light modulators operate by changing the amount of light reflected in the surface normal direction, i.e., the direction normal to the substrate surface. This may be achieved by varying the variable air gap, which alters the optical properties of the device.
Compared with other modulation means, such as a laser, micromechanical modulators are limited in terms of modulation frequency. However, the micromechanical modulators are less expensive to implement and are readily fabricated on silicon substrates facilitating integration with silicon based electronics. Further, unlike the typical semiconductor laser, micromechanical modulators operate in a surface normal manner. This is an attractive feature since a device which operates in this manner requires less wafer space than a device, such as a typical semiconductor laser, in which the operating cavity is formed in the plane of the wafer. Many thousands of surface normal modulators may be formed on a single wafer, minimizing cost. Thus, where the operating frequency is limited, the micromechanical modulator may be the modulation device of choice.
One such micromechanical modulator has been described by Aratani et al. in "Process and Design Considerations for Surface Micromachined Beams for a Tuneable Interferometer Array in Silicon, " Proc. IEEE Microelectromech Workshop, Ft. Laud., Fla., Feb. 7-10, 1993 at 230-35. This article, and all other articles referenced in this specification are herein incorporated by reference in their entirety. Aratani's modulator is described as having a diaphragm mirror consisting of a polysilicon/silicon nitride multilayer supported by thin beams over a substrate, also partially mirrored by a polysilicon/silicon oxide multilayer. As a voltage is applied between the membrane and the substrate, the membrane is pulled toward the substrate. The device is said to behave as a Fabry-Perot interferometer wherein, given two mirrors having equal reflectivity, the reflectivity of the device approaches zero at the resonant wavelength of the cavity. As the membrane moves, altering the cavity, the reflectivity of the device rises. The change in reflectivity modulates the optical signal. While a large change in reflectivity is said to be achieved, the optical bandwidth of the optical resonator based modulator is limited. The contrast ratio of such a device falls off sharply as the wavelength of the incident light varies from the resonant wavelength of the device.
A second micromechanical modulator was described by Solgaard et al in "Deformable Grating Optical Modulator, " Optics Lett 17(9) 688-90 (1992). This modulator was described as having a reflection phase grating of silicon nitride beams which is coated with metal and suspended over a substrate which is also coated with metal. An air gap separates the grating and substrate. In the absence of a biasing voltage, the path length difference between the light reflected from the grating beams and that reflected from the substrate is equal to the wavelength of the incoming light. These reflections are therefore in phase, and the device reflects the light in the manner of a flat mirror. When a voltage is applied between the beams and the substrate, the beams are brought in contact with the substrate. The total path length difference between the light reflected from the grating beams and that reflected from the substrate changes to one half of the wavelength of the incident light. In this case, the reflections interfere destructively, causing the light to be diffracted.
The deformable grating optical modulator does not achieve a low reflectivity state. Rather, it switches to a diffracting state. In the diffracting state, incident light is scattered into higher-order diffraction modes of the grating, so that the amount of light reflected into the zero order (surface-normal) mode is minimized. Such diffraction may be an undesirable aspect of the deformable grating optical modulator. If the numerical aperture of the incoming fiber or detection system is large enough to pick up the higher order diffraction modes, a degradation in contrast will result. Further, if this device is implemented in a system using arrays of optical beams or fibers, a significant crosstalk may be introduced.
Accordingly, there is a need for a micromechanical modulator which provides high contrast modulation for optical signals over a range of wavelengths, in particular from 1300 to 1550 nanometers (nm).